The invention relates to a sensor for the measurement of the refractive index of a fluid and/or the phase boundary between two fluids with visible or invisible light, which sensor comprises a light guide in the form of a rod with a refractive index greater than that of the fluid or fluids respectively, one end of the rod being connected to both a light source and a detection element sensitive to reflected light and the other end of the rod being reflective, and a cladding around the rod being provided with one or more cut-outs.
A sensor of this type is described in the German Offenlegungsschrift No. 3,012,328.
It is known that the refractive index of a fluid changes with the density of the said fluid and that the concentration of an acid in solution can be determined on the basis of a refractive index measurement, which may, for example, be of importance in the continuous or discontinuous monitoring of the state of charge of a battery in which sulphuric acid is used as electrolyte. A sensor of this type is described in "Patents Abstracts of Japan" vol. 5, No. 157, page 83 (829), Oct. 8, 1981. This known sensor is provided with a reflective membrane only at its end face projecting into the liquid, the cladding of the rod which consists of a super-transparent quartz glass is not coated. A disadvantage of this sensor is that a sufficiently accurate measuremnet can be achieved only if the liquid level remains constant.
Moreover, it is known that the position of a liquid level in a tank or vessel can be determined by measuring the light which is lost through openings in the cladding of a light guide which is inserted into the liquid. A measurement device which is suitable for this purpose is described in the said German Offenlegungsschrift 3,012,328. The sensor used in the latter is constructed in the form of an optical fibre with a refractive index greater than that of the surroundings. One end of the fibre is connected to a light source and to a detection element which is sensitive to reflected light, while the other end of the rod is reflecting. Around the fibre there is disposed a cladding, the refractive index of which is lower than that of the fibre. The cladding is provided with one or more long cut-outs which extend with their longitudinal direction in the axial direction of the fibre. Opposite the cut-outs there is located a light absorbing plastic-material film which is pressed in the liquid by hydrostatic pressure against the cut-out. Due to the presence of the light-absorbing film this sensor is certainly relatively insensitive to contamination, but disadvantages are the vulnerability, the complexity and the fact that only analogue (not digital) measurements can be made with the result that, for example, a drift in the electronic section of the device results in inaccuracies.